Reflective microcells for electrophoretic displays and methods of making the same

ABSTRACT

A polymeric film includes a plurality of tapered microcells containing a dispersion of a first group and a second group of charged particles. The first group and second group of charged particles having opposite charge polarities. The tapered microcells include a wall and at least a portion of the wall is configured to repel the first group of charged particles. Also provided is a method of making a laminate for an electrophoretic display comprising embossing a plurality of tapered microcells through a layer of polymeric film and into a release sheet to form an embossed film; laminating the embossed film to a layer of conductive material on a protective sheet to form a laminated film; removing the release sheet from the polymeric film to form an opening to an interior of each microcell of the laminated film; filling the microcells with a dispersion fluid; and sealing the microcells.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/014,123, filed Sep. 8, 2020, which is a divisional of U.S. patentapplication Ser. No. 16/015,337 filed Jun. 22, 2018, now U.S. Pat. No.10,802,373, which claimed priority to U.S. Provisional Application Ser.No. 62/524,640, filed Jun. 26, 2017. The entire disclosures of theaforementioned applications are herein incorporated by reference.

FIELD OF INVENTION

This invention relates to electrophoretic displays. More specifically,in one aspect this invention relates to improved microcells containingelectrophoretic fluid for electrophoretic displays. In another aspect,this invention relates to methods of making improved microcells forelectrophoretic displays.

BACKGROUND OF THE INVENTION

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a drive scheme which only drives pixels to their two extremeoptical states with no intervening gray states.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC. and related companies describe various technologiesused in encapsulated and microcell electrophoretic and otherelectro-optic media. Encapsulated electrophoretic media comprisenumerous small capsules, each of which itself comprises an internalphase containing electrophoretically-mobile particles in a fluid medium,and a capsule wall surrounding the internal phase. In a microcellelectrophoretic display, the charged particles and the fluid are notencapsulated within microcapsules but instead are retained within aplurality of cavities formed within a carrier medium, typically apolymeric film. See, for example, International Application PublicationNo. WO 02/01281, and published US Application No. 2002/0075556. Thetechnologies described above may be found, for example, in these patentsand applications:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Microcell structures, wall materials, and methods of forming        microcells; see for example U.S. Pat. Nos. 6,672,921; 6,751,007;        6,753,067; 6,781,745; 6,788,452; 6,795,229; 6,806,995;        6,829,078; 6,833,177; 6,850,355; 6,865,012; 6,870,662;        6,885,495; 6,906,779; 6,930,818; 6,933,098; 6,947,202;        6,987,605; 7,046,228; 7,072,095; 7,079,303; 7,141,279;        7,156,945; 7,205,355; 7,233,429; 7,261,920; 7,271,947;        7,304,780; 7,307,778; 7,327,346; 7,347,957; 7,470,386;        7,504,050; 7,580,180; 7,715,087; 7,767,126; 7,880,958;        8,002,948; 8,154,790; 8,169,690; 8,441,432; 8,582,197;        8,891,156; 9,279,906; 9,291,872; and 9,388,307; and U.S. Patent        Applications Publication Nos. 2003/0175480; 2003/0175481;        2003/0179437; 2003/0203101; 2013/0321744; 2014/0050814;        2015/0085345; 2016/0059442; 2016/0004136; and 2016/0059617;    -   (d) Methods for filling and sealing microcells; see for example        U.S. Pat. Nos. 7,144,942 and 7,715,088;    -   (e) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (f) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318 and 7,535,624;    -   (g) Color formation and color adjustment; see for example U.S.        Pat. Nos. 7,075,502 and 7,839,564;    -   (h) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600 and 7,453,445;    -   (i) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348; and    -   (j) Non-electrophoretic displays, as described in U.S. Pat. No.        6,241,921 and U.S. Patent Applications Publication No.        2015/0277160; and applications of encapsulation and microcell        technology other than displays; see for example U.S. Patent        Application Publications Nos. 2015/0005720 and 2016/0012710.

A microcell electrophoretic display typically does not suffer from theclustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.

An electrophoretic display normally comprises a layer of electrophoreticmaterial and at least two other layers disposed on opposed sides of theelectrophoretic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electrophoretic display, which is intendedfor use with a stylus, print head or similar movable electrode separatefrom the display, only one of the layers adjacent the electrophoreticlayer comprises an electrode, the layer on the opposed side of theelectrophoretic layer typically being a protective layer intended toprevent the movable electrode damaging the electrophoretic layer.

Electro-optic displays, including electrophoretic displays, can becostly; for example, the cost of the color LCD found in a portablecomputer is typically a substantial fraction of the entire cost of thecomputer. As the use of such displays spreads to devices, such ascellular telephones and personal digital assistants (PDA's), much lesscostly than portable computers, there is great pressure to reduce thecosts of such displays. The ability to form layers of electrophoreticmedia by printing techniques on flexible substrates, as discussed above,opens up the possibility of reducing the cost of electrophoreticcomponents of displays by using mass production techniques such asroll-to-roll coating using commercial equipment used for the productionof coated papers, polymeric films and similar media.

Current electrophoretic displays may also suffer from inefficientreflectance in a white optical state. For example, referring to FIG. 1 ,a plurality of cubic microcells 10 embossed in a polymeric film arefilled with an electrophoretic fluid containing a black pigment 12 andwhite pigment 14. The microcells 10 may be incorporated as a layer in ablack and white electrophoretic display. When one or more pixels of thedisplay are displaying a white optical state (as viewed from above inFIG. 1 ), a substantial amount of light may be transmitted through thewhite pigment layer 14 instead of being reflected back to the observer.The light entering the microcell 10 may be lost and likely absorbed bythe black pigment layer 12. The light loss may contribute to dull colorstates

Thus, there is a need for microcell designs for electrophoretic displayswith improved reflectance during certain optical states, such as a whiteoptical state.

SUMMARY OF THE INVENTION

It is one aspect of the present invention to provide a polymeric filmcomprising a plurality of tapered microcells containing a dispersion ofa first group and a second group of charged particles. The first groupof charged particles may have a charge polarity opposite to a chargepolarity of the second group of charged particles. The taperedmicrocells include a wall and at least a portion of the wall isconfigured to repel the first group of charged particles.

It is another aspect of the present invention to provide a method ofmaking a laminate for an electrophoretic display comprising embossing aplurality of microcells having a tapered geometry through a layer ofpolymeric film and into a release sheet to form an embossed film;laminating the embossed film to a layer of conductive material on aprotective sheet to form a laminated film; removing the release sheetfrom the polymeric film to form an opening to an interior of eachmicrocell of the laminated film; filling the microcells with adispersion fluid; and sealing the microcells.

These and other aspects of the present invention will be apparent inview of the following description.

BRIEF DESCRIPTION OF THE FIGURES

The drawing Figures depict one or more implementations in accord withthe present concepts, by way of example only, not by way of limitations.In the figures, like reference numerals refer to the same or similarelements.

FIG. 1 is a side cross-sectional view of a series of microcellscontaining black and white pigment in a dispersion fluid.

FIG. 2 a is a side cross-sectional view of a microcell according to afirst embodiment of the present invention in a white optical state.

FIG. 2 b is a side cross-sectional view of the microcell of FIG. 2 a ina black optical state.

FIG. 3 a is a plan view of four microcells according to anotherembodiment of the present invention.

FIG. 3 b is a plan view of six microcells according to a yet anotherembodiment of the present invention.

FIG. 3 c is a plan view of three microcells according to a yet anotherembodiment of the present invention.

FIG. 4 is a side cross-sectional view of a schematic of anelectrophoretic display incorporating the microcells of FIG. 2 a.

FIG. 5 is a side cross-sectional view of an embossed polymeric film andrelease sheet used in a method according to another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent to those skilledin the art that the present teachings may be practiced without suchdetails.

The present invention seeks to provide a microcell design that improveslight reflectance and minimizes light loss. The microcell designaccording to various embodiments of the present invention may include atapered geometry to provide angled reflective walls that repel black ordark pigment. The various embodiments of the present invention maysubstantially improve the reflectivity when the microcell is displayingan extreme optical state, such as a white state. Increasing the whitestate may improve display visibility in low light conditions and mayimprove the color gamut of an electrophoretic display when a filmcomprising the microcells according to the present invention arecombined with a color filter array (CFA).

According to one embodiment of the present invention illustrated inFIGS. 2 a and 2 b , the microcell 10 may contain a white pigment 14 anda black pigment 12 in a transparent fluid, wherein the microcell 10 hasa tapered form, such as an inverted pyramid with the bases of thepyramids facing the viewer and the peaks of the pyramids furthest fromthe viewer. By providing a tapered geometry, the black pigment 12 willbe located in the apex of the microcell 10 when a pixel comprising themicrocell 10 has been switched to the white state (FIG. 2 b ).Therefore, unlike a cubic geometry, such as the microcell design of FIG.1 , the black pigment 12 in the tapered microcell of FIG. 2 b has asmaller area on the viewer side than the viewing area of the whitepigment 14 thereby reducing the likelihood of light loss from absorptionby the black pigment 12.

The geometry of the microcells according to the various embodiments ofthe present invention also facilitates reflectance. For example, in FIG.2 b , at least a portion of the light 16 that passes through the whitepigment 14 may be reflected off the walls 11 of the microcell 10 andback through the white pigment layer 14 to the viewer. As would beunderstood by those of skill in the art, light 16 represents only asingle potential path for a portion of the light entering the microcell10 and is not intended to suggest a path travelled by all light enteringthe microcell 10 through the white pigment 14. By using a polymericmaterial with a high reflective property to form the walls 11 of themicrocells 10, light that is scattered through the microcell 10 willhave several points from which the light may be reflected back out tothe viewer. The angled walls 11 of the microcell 10 are preferablyspecular/mirrored, such that the walls of the microcell form aretroreflector. In another embodiment, the walls 11 of the microcell 10may be provided in the form of diffuse reflectors instead ofspecular/mirrored surfaces. This may be accomplished by embossing themicrocells 10 into a polymeric film filled with a reflective filler suchas titania.

The geometry of the microcells may be provided in a variety of shapes.For example, referring to the plan views of the various embodimentsillustrated in FIGS. 3 a, 3 b, and 3 c , the microcell geometry may be afour-sided, three-sided, or six-sided pyramid. The geometry is notlimited to a pyramid structure. For example, the geometry may beprovided in the form of a cone or a triangular prism; however,equilateral polygonal pyramids are preferred as the geometry allows forthe close packing of microcells having similar angled walls within thedisplay area. Furthermore, the apex of the tapered geometry of themicrocells may optionally be truncated or hemispherical. However,hemispherical geometries are less preferred because such geometrieswould not pack the black or other colored pigment to as small an areaaway from the viewer.

Providing a wall angle for the microcell geometry that is steep, i.e. amore acute angle at the apex, may inhibit the black pigment fromadhering to the wall and minimize the exposed area of the pigment packedinto the apex. Furthermore, it is likely that steeper walls would resultin more uniform coating of the viewing surface with pigment. Forexample, when switching from a black optical state to a white opticalstate, the white pigment is initially packed in the apex of themicrocell and must then migrate from the apex to the viewing surface ofthe microcell. If the microcell is too shallow, the vertical distancetravelled by the white pigment is short relative to the lateraldimension of the viewing area. For relatively steeper walls, the ratioof lateral to vertical distance travelled by the pigment is lower. Ahigh ratio is likely to result in a thicker pigment coating in thecenter of the viewing area and a thinner, more transmissive coatingabout the perimeter of the viewing area upon switching the opticalstate. The ratio of lateral to vertical movement of the pigment shouldbe selected to promote substantially uniform coverage of the pigmentacross the entire viewing area of each microcell. Other factors may alsobe considered when selecting the geometry of the microcell. For example,a shallow geometry will promote reflectivity by providing a shorter pathfor the light to be reflected. The dimensions of the microcell geometrymay also be selected based on the desired display resolution, contrast,and switching speed of the electrophoretic display. In a preferredembodiment, the depth of the microcells is 20 to 50 microns.

Providing the microcells with a tapered geometry may also result in anincrease of optically active surface area of the electrophoreticdisplay. Referring again to FIG. 1 , increasing the active surface ofthe electrophoretic display requires decreasing the vertical wallthickness of the cubic microcells 10 or incorporating a microlens in themicrocell. However, manufacturing vertical walls for a microcell becomesincreasingly difficult as the wall thickness decreases because of therisk of portions of the microcells being torn from the film andremaining on the embossing tool during the embossing process. Also, theuse of a microlens may result in a decreased viewing angle. The use of atapered geometry for the microcell therefore offers an easiermanufacturing method that increases the potential optically activesurface area without compromising the viewing angle range of theelectrophoretic display.

Referring to FIG. 4 , for example, a polymeric film embossed withmicrocells 10 having a tapered geometry may be incorporated in anelectrophoretic display 30. As would be understood by those of skill inthe art, FIG. 4 is not drawn to scale and is a schematic representationof the cross-section of a laminated electrophoretic display. A polymericfilm 18 embossed with a plurality of sealed microcells 10 may belaminated between a series of pixel electrodes 22 and a continuous frontelectrode 20 that is light-transmissive conductive material, such as athin layer of indium tin oxide (ITO). The pixel electrodes 22 may beprovided in the form of an array of thin film transistors (TFT) on abackplane 28. The top layers of the laminated display 30 furthercomprises a protective light-transmissive layer 24, such as PET, and anoptional CFA 26 comprising red (R), green (G), and blue (B) areas thatis also light-transmissive. Each of the microcells 10 are filled with adispersion fluid containing charged white pigment 14 and charged blackpigment 12. Therefore, excluding the optional CFA 26 will provide ablack and white display. An adhesive layer may be incorporated betweenone or more pairs of adjacent layers described above, so that the layersmay be laminated together.

In an alternative embodiment of an electrophoretic display, the locationof the single continuous electrode layer and the pixel electrodes may bereversed, such that the single continuous electrode layer is located onthe backplane and the pixel electrodes are located on the viewing sideof the microcells. In this embodiment, it is not necessary for thesingle continuous electrode layer to be light-transmissive; however, thepixel electrodes must be light-transmissive. In this arrangement, it maybe possible to provide colored pixel electrodes, so that the pixelelectrodes may simultaneously serve as the CFA.

In another embodiment of the present invention, a method of making thetapered microcells is provided. As is known by those of skill in the artof microcell formation, embossing techniques are typically used whereina tool, such as an embossing cylinder having a pattern in the shape ofthe microcells on its surface, is rolled onto a polymeric film. Afterembossing, the microcells are filled with a dispersion containing thecharged pigment. To seal the microcells, a crosslinkable oligomeric ormonomeric fluid may be coated over the filled microcells. An alternativesealing step may include laminating a sealant layer over the cups.

Referring to FIG. 5 , the most preferred method of making and sealingthe microcells according to the present invention comprises embossingthe microcells in a polymeric film with a plurality of microcells havinga tapered geometry, laminating the embossed polymeric film to acontinuous front electrode layer, forming openings in the microcells,filling the interior of the microcells with a dispersion fluid throughthe small openings, and sealing the microcells.

The embossing step of the preferred method may comprise embossing anarray of microcells 10 having a tapered geometry into a polymeric film32, such as polyester, that is laminated to a release sheet 34. The filmshould be highly reflective by metallizing or incorporating reflectiveadditives in the polymeric film, for example. If the polymeric film ismetallized prior to embossing, the metal layer would most likely becomediscontinuous at all the edges of the microcell thereby avoidingelectrical shorts between the front and rear electrodes of the display.A non-conductive reflective coating designed for constructiveinterference may also be applied to the film, such as the commerciallyavailable films used to improve emissive display backlight efficiencyknown to those of skill in the art, for example. The non-conductivereflective coating is preferably applied after embossing because thecoatings are typically composed of oxides that may not surviveembossing.

In a preferred embodiment of the present invention, the reflectivecoating on the embossed polymeric film may be a dielectric mirror. Asknown to those of skill in the art, a dielectric mirror comprises aplurality of thin layers of dielectric material deposited on asubstrate. The reflectivity properties of the dielectric mirror dependon the type of dielectric material and the thickness of the coating.Various thin-film deposition methods may be employed to manufacture thedielectric mirror, such as physical vapor deposition (e.g. evaporativedeposition and ion beam assisted deposition), chemical vapor deposition,ion beam deposition, molecular beam epitaxy, and sputter deposition.Dielectric materials used to form the dielectric mirror include, but arenot limited to, aluminum, magnesium fluoride, silicon dioxide, tantalumpentoxide, zinc sulfide (n=2.32), and titanium dioxide (n=2.4).

To further promote reflectivity of the microcell walls, the variousembodiments of the present invention may include features to prevent theblack pigment from adhering to the walls of the microcell. In order tolimit the presence of the black pigment to either spreading across thefront viewing surface or packing into the apex of the microcell, thewalls may be surface treated to repel the black pigment. For example,the microcell walls may be coated with a fluorinated polymer or otherlow surface energy material. Alternatively, after metallizing thesurface of the microcells, the walls of the microcells may be treatedwith the same chargeable groups used to form the electrophoretic blackpigment. If the microcell walls have a charge polarity that is similarto the charge polarity of the black pigment, the microcell walls willrepel the black pigment.

In one embodiment, the metallized surface of the embossed polymeric filmmay include reactive sites that may be reacted with a reagent having asilane moiety bonded to one or more polar groups and/or one or morepolymeric/polymerizable groups. The reactive sites may be hydroxylgroups, amine groups, carboxylic acid groups or derivatives thereof(e.g., amides or esters), alcohol or phenol groups or halogens,depending on the chemical functionality of the material used to providethe microcells walls with a reflective surface. The reactive sites mayalso be planted onto the surface of the microcell walls by conventionalmeans or by special treatment such as hydration as described in U.S.Ser. No. 13/149,599 filed on May 31, 2011, the content of which isincorporated herein by reference in its entirety.

The polar group of the reagent may contribute charge to the microcellwall surface. For example, polar groups such as —NH— may contribute apositive charge and polar groups such as —OH or —COOH may contribute anegative charge. The polymeric/polymerizable group includes, but is notlimited to, vinyl, acrylate, methacrylate groups or the like.

Reactive agents may include, but are not limited toN-(3-acryloxy-2-hydroxypropyl)-3-aminopropyltriethoxysilane (Gelest),3-(N-allylamino)propyltrimethoxysilane (Gelest),3-(N-styrylmethyl-2-aminoethylamino)-propyltrimethoxysilane (Gelest) orvinylbenzyl aminoethylaminopropyl-trimethoxysilane (Z-6032, by DowCorning).

The silane coupling reaction of the reagent to the microcell surface maybe initiated by first hydrolyzing the silane moiety to form a reactivesilanol group (Si—OH) which can subsequently bond with hydroxyl groupsat the surface of the embossed film via a condensation reaction.

To prevent the white pigment having an opposite charge polarity fromstrongly adhering to the microcell walls, a steric stabilizing layer maybe added to the walls in the same manner as is used on the blackpigment. For example after the silane coupling reaction, thepolymeric/polymerizable group may be polymerized if necessary with oneor more types of monomers, oligomers or polymers, and combinationsthereof, to form polymer stabilizers. The polymer stabilizers aredesired to create a steric barrier of about 1 nm to about 50 nm,preferably about 5 nm to about 30 nm, and more preferably about 10 nm toabout 20 nm, in thickness, on the microcell wall surface.

Suitable polymers, in the context of the present invention, may include,but are not limited to, polyethylene, polypropylene, polyacrylate,polyurethane, polyester or polysiloxane. Suitable monomers include, butare not limited to, lauryl acrylate, lauryl methacrylate, 2-ethylhexylacrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate,n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate andn-octadecyl methacrylate. Selection of the material for the polymerstabilizers would depend on the compatibility of the material with thesolvent used in the electrophoretic fluid.

The combined thickness of the polymeric film 32 and release sheet 34should be greater than the desired final dimensions of the geometry ofthe microcells. The polymeric film 32 should have a depth that is lessthan the height of the corresponding pattern on the embossing tool toensure that the embossing tool penetrates through the polymeric film 32and into the release sheet 34 during embossing. The release sheet 34should have approximately the same elastic modulus of the polymeric film32 and a sufficient thickness, so that the embossing cylinder will notbe damaged by the opposing cylinder tool surface. In a preferredembodiment, the release sheet may comprise silicone coated polyethyleneterephthalate.

The laminating step of the preferred method may comprise laminating aprotective PET layer, optional CFA and front plane electrode film (ITO),such as layers 24, 26, and 20 in FIG. 4 , onto the open ends of themicrocells, e.g. the base of the pyramidal microcells 10 in FIG. 5 . Anelectrical passivation layer may also be included between the microcellsand the PET-ITO layer, as well as an adhesive layer. It is preferred tolaminate the PET-ITO layer to the polymeric film prior to filling andsealing the microcells, so that the pattern of microcells includeclosely packed cells to maximize the active area fraction of thedisplay.

The forming of openings in each of the microcells may compriseseparating the release sheet 34 from the polymeric film 32 to remove thebottom portion of the embossed microcells 10, thereby forming a smallhole at the bottom of each of the microcells 10. The width of the holesshould be large enough to allow easy access to the interior of themicrocells for the dispersion fluid, but of minimal size to facilitatesealing after filling.

The filling step of the preferred method may be achieved by varioustechniques.

In one method, the microcells may be filled by first evacuating theinteriors of the microcells, such as by placing the laminated polymericfilm 32 and PET-ITO layer with the release sheet 34 removed in a vacuumchamber. After applying a vacuum to evacuate the microcells of gas, thedispersion fluid may be applied onto the surface of the polymeric film32 containing the small holes immediately followed by release of thevacuum to draw the dispersion fluid into the microcells. To minimize thepotential for solvent evaporation in the dispersion fluid, it ispreferred to place the combined polymeric film 32 and PET-ITO layer in avacuum chamber having as small a volume as possible, i.e. slightlylarger than the volume of the combined polymeric film 32 and PET-ITOlayer, and releasing the vacuum as soon as a sufficient volume ofdispersion fluid for filling the microcells has been applied to thepolymeric film 32.

Another method of filling the microcells may comprise immersing thelaminated polyester film 32 and PET-ITO layer with the release sheet 34removed in an ultrasonic bath filled with the dispersion fluid. Theultrasonic agitation would drive the gas out of the microcells to bereplaced by the dispersion fluid. The bath may be held under slightvacuum to accelerate the process if necessary. Ultrasonic agitation is apreferred filling method because of the potential for being scalable toa continuous process.

Yet another filling method may comprise filling the microcells with asolvent vapor that has a boiling point below ambient temperature, forexample, but above the pour point or freezing temperature of thedispersion fluid. The laminated polymeric film 32 and PET-ITO layer withthe release sheet 34 removed may then be immersed in the dispersionfluid and subsequently cooled to a temperature below the boiling pointof the solvent vapor in the microcells. This would cause the solventvapor to condense and draw the dispersion fluid into the microcells. Thesolvent vapor is preferably miscible in the dispersion fluid.

When the microcells are filled with the dispersion fluid, a sealant thatis preferably immiscible in the dispersion fluid may be used to seal themicrocells. A lamination adhesive that simultaneously meets theelectrical, optical, and mechanical requirements of the electrophoreticdisplay and has low solvent permeability may be used to seal themicrocells and form a front plane laminate (FPL) comprising themicrocell design of the present invention. Alternatively, a separatesealant may be used to seal the microcells prior to applying a layer oflamination adhesive and optional releasable sheet for forming an FPL.Because the openings in the polymeric film 32 are small relative to theoverall area of the polymeric film, the surface of the polymeric film 32will provide an adequate area for wetting and adhesion with the sealant.Because the sealant is applied to the rear non-viewing surface of theelectrophoretic display of this preferred method, the sealant is lesslikely to interfere with the optical properties of the display.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

We claim:
 1. A method of making a laminate for an electrophoreticdisplay, the method comprising: coupling a layer of polymeric film to arelease sheet with an adhesive layer; embossing a plurality of taperedmicrocells through the layer of polymeric film, through the adhesivelayer, and into the release sheet to form an embossed film, the embossedfilm having a top side with wider holes and a bottom side adjacent theadhesive layer having smaller holes at the bottom of each microcell;laminating the top side of the embossed film to a layer of conductivematerial on a protective sheet to form a laminated film; removing therelease sheet from the bottom side of the embossed film to form aplurality of openings in the tapered microcells, thereby providingaccess through the smaller holes to an interior of each microcell of thelaminated film; filling the interiors of the microcells with adispersion fluid through the smaller holes; and sealing the microcells.2. The method of claim 1, wherein the polymeric film includes ametallized surface.
 3. The method of claim 1, wherein the polymeric filmcomprises a reflective additive.
 4. The method of claim 1, wherein acolor filter array is located between the protective sheet and the layerof conductive material.
 5. The method of claim 1, wherein the fillingstep comprises: placing the laminated film having the opening to theinterior of each microcell in a vacuum chamber, evacuating the interiorof each microcell to create a vacuum within the vacuum chamber, applyingthe dispersion fluid to the opening of each microcell, and releasing thevacuum within the vacuum chamber.
 6. The method of claim 1, wherein thefilling step comprises immersing the laminated film having the openingto the interior of each microcell in an ultrasonic bath filled with thedispersion fluid.
 7. The method of claim 1, wherein the filling stepcomprises: filling the interior of each microcell with a solvent havinga boiling point above a pour point or freezing point of the dispersionfluid; immersing the laminated film having the opening to the interiorof each microcell in the dispersion fluid; and lowering the temperatureof the solvent below the boiling point of the solvent to draw thedispersion fluid into the interior of each microcell.
 8. The method ofclaim 1, wherein the sealing step comprises covering the opening of eachmicrocell with a sealant.
 9. The method of claim 1, wherein the taperedmicrocells have an inverted pyramid form.
 10. The method of claim 1,wherein the tapered microcells have an inverted cone form.
 11. Themethod of claim 1, wherein the tapered microcells have an invertedtriangular prism form.